1. Field of the invention
The invention relates to a semiconductor process and, in particular, to a method for forming oxides on buried N.sup.+ -type regions in a flash memory cell fabrication process in order to provide an etching stop function during self-aligned MOS device etching.
2. Description of the Prior Art
Referring to FIG. 1, a memory cell includes field oxides 10, first poly-silicon layers 12 and second polysilicon layers 14 formed in order using a conventional flash memory cell manufacturing process. Then, self-aligned MOS etching is performed on the second poly-silicon layers 14, thereby forming floating gates. Furthermore, oxides (not shown) are formed on buried N.sup.+ -type regions 16 (hereafter BN) which serve as etching stop regions during the above-mentioned self-aligned MOS etching.
In the conventional process, in order to prevent damage caused by etching, extremely thick oxides having an etching stop function are formed on doped regions of high concentration. The doped regions are formed by increasing the impurity concentration of BN regions through ion implantation, raising the temperature to about 900.degree. C., and annealing the substrate with buried N.sup.+ -type regions for about 60 minutes. The oxides are deposited in a high temperature vapor environment for about 20 minutes wet. The principle of forming extremely thick oxides on doped regions of high concentration is described in "Thermal Oxidation of Heavily Doped Silicon" disclosed by B. E. Deal et al. and "Thermal Oxidation of Heavily Phosphorus-Doped Silicon" disclosed by C. P. Ho et al.
However, in a sub-micron semiconductor fabrication process, since the area of a BN region 16 is only about 0.6 .mu.m.times.0.6 .mu.m as shown in FIG. 1 and the two sides of the BN region define the edges of the field oxides, the birds beaks 23a, 23b of field oxides 22a and 22b on the substrate 20 shown in FIG. 2 cover a part of the regions to be doped during ion implantation so that the dopant concentration on the two sides of a BN region is less than the dopant concentration in the center. As a result, the oxide on the center part of a BN region is thicker than those on the two sides of the BN region, resulting in the BN region at the edges of the field oxides 22a and 22b being too readily etched during subsequent etching, thereby creating a trench 24 as shown in FIG. 2.